In paper machine headboxes of the present type, the profile bar on the top slice lip serves to "modulate" the flow of the pulp stock through the horizontal slice opening in the transverse direction, i.e. to allow a control of the grammage of the paper in the transverse direction of the paper web. The adjustment of the profile bar relative to the top slice lip is carried out by means of adjusting knobs or the like, which can be reached by an operator from a bridge that transverses the paper making machine. In connection with these adjusting knobs or corresponding means there are provided reading instruments showing the recorded position of the profile bar. However, due to unavoidable play in the transmission between the profile bar and the reading instruments, the indication is unsafe wherefore the recorded position can deviate from the real position of the profile bar. The deviation often is greater than can be tolerated.